The present invention relates to a cooling apparatus for performing cooling throughout the year and a control method therefor and, more particularly, to a control method when an outside temperature is low.
FIG. 1 shows a basic arrangement of a cooling apparatus. A refrigeration cycle is mainly constituted by four components, i.e., a compressor 121, a condenser 122, an expansion valve 123, and an evaporator 124. In an air cooling type air conditioner, an evaporator is installed indoors, and a condenser is installed outdoors. A refrigerant is sealed in the refrigeration cycle, and the refrigerant is circulated among the components, so that indoor heat can be exhausted outside a room.
The function of this refrigeration cycle will be explained below. In the evaporator 124, a low-pressure refrigerant liquid takes indoor heat away and is evaporated to be a gas. The refrigerant gas vaporized in the evaporator is sent to the compressor 121, and is compressed to a high-temperature, high-pressure gas. The obtained gas is supplied to the condenser 122. In the condenser, heat is exhausted to outer air in an opposite way to the evaporator, thus converting the gas into a high-pressure liquid. The high-pressure liquid is supplied to the expansion valve 123, and is expanded to a low-temperature, low-pressure liquid. The obtained liquid is supplied again to the evaporator. In this manner, this cycle is repeated. More specifically, upon operation of the compressor, the refrigerant gas having a lower saturation temperature than a room temperature is compressed to a gas having a higher saturation temperature than that of outer air, thereby exhausting indoor heat outside a room.
However, when an outside temperature is low, if the cooling apparatus shown in FIG. 1 is operated as in a case wherein an outside temperature is high, exchange heat of the condenser is enhanced, and a condensing pressure as a refrigerant pressure in the condenser is decreased. When the condensing pressure is decreased, an evaporative pressure as a refrigerant pressure in the evaporator is decreased accordingly. As a result, the evaporator is frosted, or is operated under an over-dehumidified condition. If the compression power in the compressor is decreased so that the evaporative pressure is not decreased even if the condensing pressure is decreased, the flow rate of the refrigerant circulating in the refrigeration cycle is decreased. As a result, a cooling capacity is decreased or a degree of superheat is increased.
In order to eliminate the above problems, in the conventional apparatus, a bypass is arranged along a path extending from the compressor to the expansion valve, and a condensing pressure control valve 226 in FIG. 2 is arranged at a merging point of the bypass and a path via the condenser. The condensing pressure control valve is a three-way valve in which a predetermined pressure of a gas is sealed. When the condensing pressure is decreased, the control valve automatically open the valve of the bypass, so that the condensing pressure is increased up to a value as high as that when the outside temperature is high. In this manner, the control valve can prevent the evaporative pressure and the cooling capacity from being decreased.
However, when the outside temperature is low, since a pressure difference between the condensing pressure and the evaporative pressure can be small, this method loses an opportunity of a low-compression power operation of the cooling apparatus. In this method, the condensing pressure is increased to increase the pressure difference between the condensing pressure and the evaporative pressure, thereby increasing compression power to operate the apparatus, resulting in a noneconomical operation method.
If the problem of the operation of the cooling apparatus shown in FIG. 1 at a low outside temperature is solved in view of keeping a sufficient refrigerant circulation amount, a method of increasing the maximum valve lift of the expansion valve 123 may be employed. The method of increasing the valve lift of the expansion valve includes a method of arranging a plurality of expansion valves parallel to each other, or a method of mounting an expansion valve having a capacity larger than a conventional value (e.g., an expansion valve for a cooling capacity of 10 RT (ton of refrigeration) is attached to an air conditioner of a cooling capacity of 5 RT). Note that "RT" is a unit indicating a cooling capacity of the cooling apparatus, and 1 RT=3,320 kcal/h. When the valve lift of the expansion valve is increased, the low condensing pressure state is maintained, and an operation with low compression power in the compressor is allowed.
However, in this operation method, the valve lift of the expansion valve is limited. If the number of expansion valves is increased too much (three or four valves are mounted), or if an expansion valve having an excessively large capacity (e.g., an expansion valve for 15 RT is attached to an air conditioner of a cooling capacity of 5 RT) is used, superheat control cannot be accurately performed when the condensing pressure is high. Thus, this method also has a limitation. When an outside temperature is decreased, the condensing pressure is excessively decreased. Even if the expansion valve is fully opened, the refrigerant circulation amount is decreased, and the degree of superheat is undesirably increased.